Color mixing illumination light unit and system using same

ABSTRACT

An illumination light unit includes at least two light sources that generate light at different wavelengths. The illumination light unit also includes a reflecting cavity having one or more reflectors and a controlled transmission mirror disposed at an output of the reflecting cavity. The controlled transmission mirror includes an input coupling element, an output coupling element and a first multilayer reflector disposed between the input and output coupling elements. At least some of the light from the light sources is reflected within the reflecting cavity by the one or more reflectors and is mixed. Light passes out of the reflecting cavity through the controlled transmission mirror. The illumination light unit may be used for illumination purposes, or as part of a backlight for illuminating a display.

RELATED APPLICATIONS

This application is related to the following applications, all of whichare incorporated herein by reference: U.S. patent application Ser. No.11/166,723, titled “OPTICAL ELEMENT FOR LATERAL LIGHT SPREADING INBACK-LIT DISPLAYS AND SYSTEM USING SAME”, filed on even date herewithand having U.S. patent application Ser. No. 11/167,003, titled “OPTICALELEMENT FOR LATERAL LIGHT SPREADING IN EDGE-LIT DISPLAYS AND SYSTEMUSING SAME”, filed on even date herewith and having U.S. patentapplication Ser. No. 11/167,001, titled “ILLUMINATION ELEMENT AND SYSTEMUSING SAME”, filed on even date herewith and having and U.S. patentapplication Ser. No. 11/167,019, titled “POLARIZATION SENSITIVEILLUMINATION ELEMENT AND SYSTEM USING SAME”, filed on even date herewithand having.

FIELD OF THE INVENTION

The invention relates to optical lighting and displays, and moreparticularly to signs and display systems that are illuminated bybacklights.

BACKGROUND

Liquid crystal displays (LCDs) are optical displays used in devices suchas laptop computers, hand-held calculators, digital watches andtelevisions. Some LCDs, for example in laptop computers, cell phones andcertain LCD monitors and LCD televisions (LCD-TVs), are illuminated frombehind using a backlight that has a number of light sources positionedto the side of the display panel. The light is guided from the lightsources using a light guide that is positioned behind the display. Thelight guide is typically configured to extract the light from the lightguide and to direct the light towards the display panel. Thisarrangement is commonly referred to as an edge-lit display, and is oftenused in applications where the display is not too large and/or thedisplayed image does not have to be very bright. For example, mostcomputer monitors are viewed from a close distance, and so do not haveto be as bright as an equivalently sized television display, which istypically viewed from a greater distance.

In larger, or brighter displays, the backlight tends to employ lightsources positioned directly behind the display panel. One reason forthis is that the light power requirements to achieve a certain level ofdisplay brightness increase with the square of the display size. Sincethe available real estate for locating light sources along the side ofthe display only increases linearly with display size, there comes apoint where the light sources have to be placed behind the panel ratherthan to the side in order to achieve the desired level of brightness.

One important aspect of the backlight is that the light illuminating thedisplay panel should be uniformly bright. Illuminance uniformity isparticularly a problem when the light sources used are point sources,for example are light emitting diodes (LEDs). In such cases thebacklight is required to spread the light across the display panel sothat the displayed image has no dark areas. In addition, in someapplications the display panel is illuminated with light from a numberof different LEDs that produce light of different colors. It isimportant in these situations that the light from the different LEDs bemixed so that the color, as well as the brightness, are uniform acrossthe displayed image.

SUMMARY OF THE INVENTION

One embodiment of the invention is directed to an optical system thathas an image-forming panel having an illumination side and a backlightunit disposed to the illumination side of the image-forming panel. Thebacklight unit includes at least first and second light sources, thefirst light source producing light at a first wavelength and the secondlight source producing light at a second wavelength different from thefirst wavelength. The backlight unit also includes a reflecting cavityhaving at least one reflecting surface and a controlled transmissionmirror. The light from the first and second light sources is reflectedwithin the reflecting cavity. The controlled transmission mirror has aninput coupling element, an output coupling element and a firstmultilayer reflector between the input and output coupling elements. Thefirst multilayer reflector is reflective for normally incident lightfrom the first and second light sources. The input coupling elementredirects at least some of the light propagating from the first andsecond light sources in a direction substantially perpendicular to thefirst multilayer reflector into a direction that is transmitted throughthe first multilayer reflector.

Another embodiment of the invention is directed to an illumination lightunit that has at least a first light source capable of generatingillumination light at a first wavelength and a second light sourcecapable of generating illumination light at a second wavelengthdifferent from the first wavelength. The illumination light unit alsoincludes a reflecting cavity having one or more reflectors and acontrolled transmission mirror disposed at an output of the reflectingcavity. The controlled transmission mirror includes an input couplingelement, an output coupling element and a first multilayer reflectordisposed between the input and output coupling elements. At least someof the illumination light from the first and second light sources isreflected within the reflecting cavity by the one or more reflectors andis transmitted out of the reflecting cavity through the controlledtransmission mirror.

The above summary of the present invention is not intended to describeeach illustrated embodiment or every implementation of the presentinvention. The following figures and detailed description moreparticularly exemplify these embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIG. 1A schematically illustrates a back-lit liquid crystal display(LCD) device that has an illumination light unit having a controlledtransmission mirror according to principles of the present invention;

FIG. 1B schematically illustrates an edge-lit LCD device that has anillumination light unit having a controlled transmission mirroraccording to principles of the present invention;

FIGS. 2A-2D schematically illustrate partial cross-sectional views ofembodiments of illumination light units having a controlled transmissionmirror according to principles of the present invention;

FIGS. 3A-3D schematically illustrate partial cross-sectional views ofdifferent embodiments of input coupling elements for a controlledtransmission mirror, according to principles of the present invention;

FIGS. 4A-4D schematically illustrate cross-sectional views of differentembodiments of output coupling elements for a controlled transmissionmirror according to principles of the present invention;

FIG. 5A schematically illustrates a cross-sectional view of anembodiment of a polarization sensitive controlled transmission mirroraccording to principles of the present invention;

FIGS. 5B and 5C schematically illustrate different embodiments ofpolarization-sensitive output coupling elements for a controlledtransmission mirror according to principles of the present invention;

FIGS. 6A-6C schematically illustrate partial cross-sectional views ofembodiments of illumination light units that use controlled transmissionmirrors according to principles of the present invention;

FIG. 7 schematically illustrates an embodiment of an arrangement ofmultiple illumination units that may be used in a backlight for adirect-lit display according to principles of the present invention;

FIG. 8A schematically illustrates an exemplary embodiment of anillumination light unit that employs a controlled transmission mirroraccording to principles of the present invention;

FIG. 8B schematically illustrates a backlight unit having anillumination light unit that directs light into a light guide, accordingto principles of the present invention;

FIGS. 9A and 9B schematically illustrate embodiments of illuminationlight units that employ controlled transmission mirrors according toprinciples of the present invention;

FIGS. 10A and 10B schematically illustrate another exemplary embodimentof light illumination units having a controlled transmission mirroraccording to principles of the present invention; and

FIGS. 11A-11C schematically illustrate additional exemplary embodimentsof light illumination units having a controlled transmission mirroraccording to principles of the present invention.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

The present invention is applicable to illuminated signs and displays,such as liquid crystal displays (LCDs, or LC displays), and isapplicable to displays that are illuminated using light sourcespositioned directly behind the display panel, known as direct-litdisplays, and to displays that are illuminated using light sourcespositioned to the side of the display panel, known as edge-lit displays.The invention is believed to be particularly useful for displays thatare illuminated by light sources of different colors. The invention isbelieved also to be applicable to systems that provide space lighting.

A schematic exploded view of an exemplary embodiment of a direct-litdisplay system 100 is presented in FIG. 1A. Such a display system 100may be used, for example, in an LCD monitor or LCD-TV. In this exemplaryembodiment, the device 100 uses a liquid crystal (LC) display panel 102,which typically comprises a layer of LC 104 disposed between panelplates 106. The plates 106 are often formed of glass, and may includeelectrode structures and alignment layers on their inner surfaces forcontrolling the orientation of the liquid crystals in the LC layer 104.The electrode structures are commonly arranged so as to define LC panelpixels, areas of the LC layer where the orientation of the liquidcrystals can be controlled independently of adjacent pixels. A colorfilter may also be included with one or more of the plates 106 forimposing color on the displayed image.

An upper absorbing polarizer 108 is positioned above the LC layer 104and a lower absorbing polarizer 110 is positioned below the LC layer104. In the illustrated embodiment, the upper and lower absorbingpolarizers 108, 110 are located outside the display panel 102. Theabsorbing polarizers 108, 110 and the display panel 102, in combination,control the transmission of light from a backlight 112 through thedisplay panel 102 to the viewer. In some exemplary embodiments, when apixel of the LC layer 104 is not activated, it does not change thepolarization of light passing therethrough. Accordingly, light thatpasses through the lower absorbing polarizer 110 is absorbed by theupper absorbing polarizer 108, when the absorbing polarizers 108, 110are aligned perpendicularly. When the pixel is activated, on the otherhand, the polarization of the light passing therethrough is rotated, sothat at least some of the light that is transmitted through the lowerabsorbing polarizer 110 is also transmitted through the upper absorbingpolarizer 108. Selective activation of the different pixels of the LClayer 104, for example using a controller 113, results in the lightpassing out of the display 100 at certain desired locations, thusforming an image seen by the viewer. The controller 113 may include, forexample, a computer or a television controller that receives anddisplays television images. One or more optional layers 109 may beprovided over the upper absorbing polarizer 108, for example to providemechanical and/or environmental protection to the display surface. Inone exemplary embodiment, the layer 109 may include a hardcoat over theabsorbing polarizer 108.

Some types of LC displays may operate in a manner different from thatdescribed above and, therefore, differ in detail from the describedsystem. For example, the absorbing polarizers may be aligned paralleland the LC panel may rotate the polarization of the light when in anunactivated state. Regardless, the basic structure of such displaysremains similar to that described above.

The backlight 112 generates illumination light and directs the lighttowards the back of the LC panel 102. The backlight 112 comprises areflecting cavity 114 that contains a number of light sources 116 forgenerating the light, or that receives light from the light sources 116.The light sources 116 may be, for example, light emitting diodes (LEDs),organic LEDs. (OLEDs), or may be other types of light sources.

The reflecting cavity 114 may include a base reflector 118 that reflectslight propagating downwards from the light sources 116 in a directionaway from the display panel 102. The base reflector 118 may also beuseful for recycling light within the display device 100, as isexplained below. The base reflector 118 may be a specular reflector ormay be a diffuse reflector. One example of a specular reflector that maybe used as the base reflector 118 is Vikuiti™ Enhanced SpecularReflection (ESR) film available from 3M Company, St. Paul, Minn.Examples of suitable diffuse reflectors include polymers, such aspolyethylene terephthalate (PET), polycarbonate (PC), polypropylene,polystyrene and the like, loaded with diffusely reflective particles,such as titanium dioxide, barium sulphate, calcium carbonate and thelike. Other examples of diffuse reflectors, including microporousmaterials and fibril-containing materials, are discussed in co-ownedU.S. Patent Application Publication 2003/0118805 A1, incorporated hereinby reference.

The reflecting cavity 114 also includes a controlled transmission mirror120 disposed between the light sources 116 and the display panel 102.The term cavity is used for an arrangement of the controlledtransmission mirror and at least one reflecting surface, where at leastsome of the light is able to reflect back and forth between thecontrolled transmission mirror and the reflecting surface.

The controlled transmission mirror 120 reflects some of the light withinthe reflecting cavity 114 and also encourages light to spread laterallywithin the cavity 114. The lateral light spreading aids in making theintensity profile of the light that exits the controlled transmissionmirror 120 uniform, so that the viewer sees a more uniformly illuminatedimage. In addition, where different light sources 116 produce light ofdifferent colors, the lateral spreading of the light results in a morecomplete mixing of the different colors, and so the viewer sees an imageof a more uniform color. The operation of the controlled transmissionmirror 120 is discussed in more detail below.

The cavity 114 may also be provided with reflecting walls 122. Thereflecting walls 122 may be formed, for example, of the same specular ordiffuse reflecting material as is used for the base reflector 118, or ofsome other type of reflecting material.

An arrangement of light management layers 124 may also be positionedbetween the cavity 114 and the display panel 102. The light managementlayers 124 affect the light propagating from the cavity 114 so as toimprove the operation of the display device 100. For example, the lightmanagement layers 124 may include a reflective polarizer 126. This maybe advantageous because the light sources 116 typically produceunpolarized light, whereas the lower absorbing polarizer 110 onlytransmits a single polarization state. Therefore, about half of thelight generated by the light sources 116 is not suitable fortransmission through to the LC layer 104. The reflecting polarizer 126,however, may be used to reflect the light that would otherwise beabsorbed in the lower absorbing polarizer 110, and so this light may berecycled by reflection between the reflecting polarizer 126 and thecavity 114. The light reflected by the reflecting polarizing 126 may besubsequently reflected by the controlled transmission mirror 120 or thelight may re-enter the cavity 114 and be reflected by the base reflector118. At least some of the light reflected by the reflecting polarizer126 may be depolarized and subsequently returned to the reflectingpolarizer 126 in a polarization state that is transmitted through thereflecting polarizer 126 and the lower absorbing polarizer 110 to thedisplay panel 102. In this manner, the reflecting polarizer 126 may beused to increase the fraction of light emitted by the light sources 116that reaches the display panel 102, and so the image produced by thedisplay device 100 is brighter.

Any suitable type of reflective polarizer may be used, for example,multilayer optical film (MOF) reflective polarizers, diffuselyreflective polarizing film (DRPF), such as continuous/disperse phasepolarizers, wire grid reflective polarizers or cholesteric reflectivepolarizers.

Both the MOF and continuous/disperse phase reflective polarizers rely onthe difference in refractive index between at least two materials,usually polymeric materials, to selectively reflect light of onepolarization state while transmitting light in an orthogonalpolarization state. Some examples of MOF reflective polarizers aredescribed in co-owned U.S. Pat. No. 5,882,774, incorporated herein byreference. Commercially available examples of MOF reflective polarizersinclude Vikuiti™ DBEF-D200 and DBEF-D400 multilayer reflectivepolarizers that include diffusive surfaces, available from 3M Company,St. Paul, Minn.

Examples of DRPF useful in connection with the present invention includecontinuous/disperse phase reflective polarizers as described in co-ownedU.S. Pat. No. 5,825,543, incorporated herein by reference, and diffuselyreflecting multilayer polarizers as described in, e.g., U.S. Pat. No.5,867,316, also incorporated herein by reference. Other suitable typesof DRPF are described in U.S. Pat. No. 5,751,388.

Some examples of wire grid polarizers useful in connection with thepresent invention include those described in U.S. Pat. No. 6,122,103.Wire grid polarizers are commercially available from, inter alia, MoxtekInc., Orem, Utah.

Some examples of cholesteric polarizers useful in connection with thepresent invention include those described in, e.g., U.S. Pat. No.5,793,456, and U.S. Patent Publication No. 2002/0159019. Cholestericpolarizers are often provided along with a quarter wave retarding layeron the output side, so that the light transmitted through thecholesteric polarizer is converted to linear polarization.

A polarization mixing layer 128 may be placed between the cavity 114 andthe reflecting polarizer 126 to aid in mixing the polarization of thelight reflected by the reflecting polarizer 126. For example, thepolarization mixing layer 128 may be a birefringent layer such as aquarter-wave retarding layer.

The light management layers 124 may also include one or more prismaticbrightness enhancing layers 130 a, 130 b. A prismatic brightnessenhancing layer is one that includes a surface structure that redirectsoff-axis light into a propagation direction closer to the axis of thedisplay. This controls the viewing angle of the illumination lightpassing through the LC panel 102, typically increasing the amount oflight propagating on-axis through the LC panel 102. Consequently, theon-axis brightness of the image seen by the viewer is increased.

One example of a prismatic brightness enhancing layer has a number ofprismatic ridges that redirect the illumination light, through acombination of refraction and reflection. Examples of prismaticbrightness enhancing layers that may be used in the display deviceinclude the Vikuiti™ BEFII and BEFIII family of prismatic filmsavailable from 3M Company, St. Paul, Minn., including BEFII 90/24, BEFII90/50, BEFIIIM 90/50, and BEFIIIT. It is possible that only onebrightness enhancing layer is used, although it is also possible to usetwo brightness enhancing layers 130 a, 130 b, with their prismaticstructures oriented at about 90° to each other. This crossedconfiguration provides control of the viewing angle of the illuminationlight in two dimensions, the horizontal and vertical viewing angles.

An exemplary embodiment of a display device 150 that includes anedge-lit display is schematically illustrated in FIG. 1B. In thisembodiment, the backlight 112 includes a light guide 152 and one or moreillumination light units 154 that generate the illumination light anddirect the illumination light into the light guide 152. The illuminationlight units 154 include a number of light sources 116 to generate theillumination light. The light sources 116 may be extended light sourcesthat emit light over an extended length. One example of an extendedlight source is a cold cathode, fluorescent tube. The light sources 116may also be effective point light sources, for example light emittingdiodes (LEDs). Other types of light sources may also be used. This listof light sources is not intended to be limiting or exhaustive, but onlyexemplary.

The illumination light unit 154 may include a reflecting cavity that isused to collect and direct light from the light sources 116 to thelightguide 152. The lightguide 152 guides illumination light from thelight sources 116 to an area behind the display panel 102, and directsthe light to the display panel 102. The light guide 152 may receiveillumination light through a single edge, or through multiple edges. Inother embodiments, not illustrated, the light may be coupled into thelight guide 152 through a light coupling mechanism other than the edgeof the light guide 152. A base reflector 156 may be positioned on theother side of the light guide 152 from the display panel 102. The lightguide 152 may include light extraction features 153 that are used toextract the light from the lightguide 152 for illuminating the displaypanel 102. For example, the light extraction features 153 may comprisediffusing spots on the surface of the light guide 152 that direct lighteither directly towards the display panel 102 or towards the basereflector 156. Other approaches may be used to extract the light fromthe light guide 152.

One exemplary embodiment of an illumination light unit 199 is nowdescribed with reference to FIG. 2A. The figure shows part of the lightunit 199, including some light sources 116 a, 116 b. A reflecting cavity118 may be formed between at least one reflecting surface 202 and acontrolled transmission mirror 200 that are arranged so that at leastsome of the illumination light produced by the sources 116 a, 116 b isreflected by both the controlled transmission mirror 200 and thereflecting surface 202. In the illustrated embodiment, the reflectingsurface 202 is positioned behind the light sources 116 a, 116 b.

The controlled transmission mirror 200 comprises a multilayer reflector204 that has a reflection spectrum such that at least some of the lightgenerated by the light sources 116 a, 116 b, when normally incident onthe multilayer reflector 204, is reflected.

An input coupling element 206 is disposed at the lower side of themultilayer reflector 204, and an output coupling element 208 is disposedat the upper side of the multilayer reflector 204. The input couplingelement 206 and the output coupling element 208 are used to change thedirection of at least some of the light entering the coupling elements206, 208, so as to couple light through the controlled transmissionmirror 200. Exemplary embodiments of input coupling elements 206 andoutput coupling elements 208 include diffusers, both surface and bulkdiffusers, and microreplicated surfaces. Examples of input couplingelements 206 and output coupling elements 208 are described in greaterdetail below. The output coupling element 208 may be the same type ofcoupling element as the input coupling element 206, for example, theinput and output coupling element 206, 208 may both be bulk diffusers,or the output coupling element 208 may be different from the inputcoupling element 206. The input and output coupling elements 206, 208may be laminated or otherwise formed integrally with the multilayerreflector 204.

The multilayer reflector 204 is generally constructed of opticalrepeating units that form the basic building blocks of a dielectricstack. The optical repeating units typically include two or more layersof at least a high and a low refractive index material. A multilayerreflector can be designed, using these building blocks, to reflectinfrared, visible or ultraviolet wavelengths and one or both of a givenorthogonal pair of polarizations of light. In general, the stack can beconstructed to reflect light of a particular wavelength, λ, bycontrolling the optical thickness of the layers according to therelationship:λ=(2/M)*D _(r),  (1)where M is an integer representing the order of the reflected light, andD_(r) is the optical thickness of an optical repeating unit. For thefirst order reflection (M=1), the optical repeating unit has an opticalthickness of λ/2. Simple quarter-wave stacks comprise a number of layersthat each have an optical thickness of one quarter of the wavelength,λ/4. Broadband reflectors can include multiple quarter-wave stacks tunedto various wavelengths, a stack with a continuous gradation of the layerthickness throughout the stack, or combinations thereof. A multilayerreflector may further include non-optical layers. For example, acoextruded polymeric dielectric reflector may include protectiveboundary layers and/or skin layers used to facilitate formation of thereflector film and to protect the reflector. Polymeric optical stacksparticularly suited to the present invention are described in publishedPCT Patent Application WO 95/17303, entitled Multilayer Optical Film andU.S. Pat. No. 6,531,230, incorporated herein by reference. In otherembodiments, the dielectric stack may be a stack of inorganic materials.Some suitable materials used for the low refractive index materialinclude SiO₂, MgF₂, CaF₂ and the like. Some suitable materials used forthe high refractive index material include TiO₂, Ta₂O₅, ZnSe and thelike. The invention is not limited to quarter-wave stacks, however, andis more generally applicable to any dielectric stack including, forexample, computer optimized stacks and random layer thickness stacks.

Reflection by a dielectric stack of light at a particular wavelength isdependent, in part, on the propagation angle through the stack. Themultilayer reflector 204 may be considered as having a reflection bandprofile (e.g., band center and bandedges) for light propagating in thestack at a particular angle. This band profile changes as the angle ofpropagation in the stack changes. The propagation angle in the stack isgenerally a function of the incident angle and the refractive indices ofthe materials in the stack and the surrounding medium. The wavelength ofthe bandedge of the reflection band profile changes as the propagationangle in the stack changes. Typically, for the polymeric materials underconsideration, the bandedge of the reflector for light at normalincidence shifts to about 80% of its normal incidence value when viewedat grazing incidence in air. This effect is described in greater detailin U.S. Pat. No. 6,208,466, incorporated herein by reference. Thebandedge shift may shift considerably further when the light is coupledinto the reflector using a medium having a refractive index higher thanair. Also, the shift in the bandedge is typically greater forp-polarization light than for s-polarization light.

The angular dependence of the reflection band profile, e.g., bandedgeshifting with angle, results from a change in the effective layerthickness. The reflection band shifts towards shorter wavelengths as theangle increases from normal incidence. While the total path lengththrough a given layer increases with angle, the change in band positionwith angle does not depend on the change in the total path lengththrough a layer with angle, θ, where the angle is measured relative toan axis 230 perpendicular to the layers of the reflector 204. Rather,the band position depends on the difference in path length between lightrays reflected from the top and bottom surfaces of a given layer. Thispath difference decreases with angle of incidence as shown by thefamiliar formula n.d.cos θ=λ, the wavelength to which a given layer istuned as a λ/4 thick layer. In the expression, n is the refractive indexof the layer material and d is the thickness of the layer.

The above description describes how the bandedge of the reflection bandprofile changes as a function of angle. As used herein, the termbandedge generally refers to the region where the multilayer reflectorchanges from substantial reflection to substantial transmission. Thisregion may be fairly sharp and described as a single wavelength. Inother cases, the transition between reflection and transmission may bemore gradual and may be described in terms of a center wavelength andbandwidth. In either case, however, a substantial difference betweenreflection and transmission exists on either side of the bandedge.

As light at the particular wavelength propagates in the stack atincreasing propagation angles (measured from the axis 230 normal to theinterface of the repeating units), the light approaches the bandedge. Inone example, at high enough propagation angles, the stack will becomesubstantially transparent to that particular wavelength of light and thelight will transmit through the stack. Thus, for a given wavelength oflight, the stack has an associated propagation angle below which thestack substantially reflects the light and another propagation angle,above which the stack substantially transmits the light. Accordingly, incertain multilayer stacks, each wavelength of light may be considered ashaving a corresponding angle below which substantial reflection occursand a corresponding angle above which substantial transmission occurs.The sharper the bandedge, the closer these two angles are for theassociated wavelength. For the purposes of the present description, theapproximation is made that these two angles are the same and have avalue of θ_(min).

The above description describes the manner in which monochromatic lightin a given stack shifts from reflection to transmission with increasingangle of propagation. If the stack is illuminated with light having amixture of components at different wavelengths, the angle, θ_(min), atwhich the reflective stack changes from being reflective to transmissiveis likely to be different for the different wavelength components. Sincethe bandedge moves to shorter wavelengths with increasing angle, thevalue of θ_(min) is lower for light at longer wavelengths, potentiallyallowing more light at longer wavelengths to be transmitted through themultilayer reflector than at shorter wavelengths. In some embodiments itmay be desired that the color of the light passing out of the controlledtransmission mirror be relatively uniform. One approach to balancing thecolor is to use an input and output coupling element that couples morelight at shorter wavelengths than at longer wavelengths into thecontrolled transmission mirror.

One example of such a coupling element is a bulk diffuser that containsscattering particles dispersed within a polymer matrix, as is discussedbelow with regards to FIGS. 3A and 4A. The scattering particles have arefractive index different from the surrounding matrix. The nature ofdiffusive scattering is that, all else being equal, light at shorterwavelengths is scattered more than light at longer wavelengths.

In addition, the degree of scattering is dependent on the differencebetween the refractive indices of the particles and the surroundingmatrix. If the difference in refractive index is greater at shorterwavelengths, then even more short wavelength light is scattered. In oneparticular embodiment of a diffusive coupling element, the matrix isformed of biaxially stretched PEN, which has an in-plane refractiveindex of about 1.75 for red light and about 1.85 for blue light, wherethe light is s-polarized, i.e., has high dispersion. The in-planerefractive index is the refractive index for light whose electric vectoris polarized parallel to the plane of the film. The out-of-planerefractive index, for light polarized parallel to the thicknessdirection of the film, is about 1.5 The refractive index for p-polarizedlight is lower than that of the s-polarized light, since the p-polarizedlight experiences an effective refractive index that is a combination ofthe in-plane refractive index and the out-of-plane refractive index.

The particles in the matrix may have a high refractive index, forexample titanium dioxide particles have a refractive index of about 2.5.The refractive index of TiO₂ varies by approximately 0.25 over the range450 nm-650 nm, which is greater than the approximately 0.1 variation forPEN over a similar wavelength range. Thus, the refractive indexdifference between the particles and the matrix changes by about 0.15across the visible spectrum, resulting in increased scattering for theblue light. Consequently, the refractive index difference between theparticles and the matrix can vary significantly over the visiblespectrum.

Thus, due to the wavelength dependence of the diffusive scatteringmechanism and the large difference in the refractive index differenceover the visible spectrum, the degree to which blue light is scatteredinto the multilayer reflector is relatively high, which at leastpartially compensates for the larger value of θ_(min), at shorterwavelengths.

Other embodiments of input and output coupling elements, for examplethose described below with reference to FIGS. 3B-3D and 4B-4D, relyprimarily on refractive effects for diverting the light. For example, acoupling element may be provided with a surface structure or holographicfeatures for coupling the light into or out of the multilayer reflector.Normal material dispersion results in greater refractive effects forshorter wavelengths. Therefore, input and output coupling elements thatrely on refractive effects may also at least partially compensate forthe larger value of θ_(min), at shorter wavelengths.

Understanding, therefore, that the light entering the controlledtransmission mirror may have a wide variation in the value of θ_(min),the following description refers to only a single value of θ_(min), forsimplicity.

Another effect that the system designer can use to control the amount oflight passing through the multilayer reflector is the selection of aBrewster's angle, the angle at which p-polarized light passes throughthe multilayer reflector without reflective loss. For adjacent isotropiclayers 1 and 2 in the multilayer reflector, having refractive indices n1and n2 respectively, the value of Brewster's angle in layer 1, θ_(B),for light passing from layer 1 to layer 2, is given by the expressiontan θ_(B)=n2/n1. Thus, the particular materials employed in thedifferent layers of the multilayer reflector may be selected to providea desired value of Brewster's angle.

The existence of the Brewster's angle for a multilayer reflectorprovides another mechanism for allowing light to pass through thereflector other than relying on the input and output coupling layers todivert the light through large angles. As the angle within thecontrolled transmission mirror is increased for p-polarized light, thereflection band substantially disappears at Brewster's angle. At anglesabove the Brewster's angle, the reflection band reappears and continuesto shift to shorter wavelengths.

In certain embodiments, it may be possible to set the value of θ_(B) forblue light to be less than θ_(min), but have θ_(B) be greater thanθ_(min) for red light. This configuration may lead to an increasedtransmission for blue light through the multilayer reflector, whichcompensates at least in part for the higher value of θ_(min) for shorterwavelength light.

At least some of the light from the light source 116 a propagatestowards the controlled transmission mirror 200. A portion of the light,exemplified by light ray 210, passes through the input coupling element206 and is incident on the multilayer reflector 204 at an angle greaterthan θ_(min) and is transmitted through the reflector 204. Anotherportion of the light, exemplified by light ray 212, is incident at theinput coupling element 206 at an angle less than θ_(min), but isdiverted by the input coupling element 206 to an angle of at leastθ_(min), and is transmitted through the multilayer reflector 204.Another portion of light from the light source 116 a, exemplified bylight ray 214, passes through the input coupling element 206 and isincident at the multilayer reflector 204 at an angle that is less thanθ_(min). Consequently, light 214 is reflected by the multilayerreflector 204 to the reflecting surface 202. The light 214 may bereflected at the reflecting surface 202 either specularly or diffusely.

In some embodiments it may be desired that the multilayer reflector 204is attached to the output coupling element 208 in a manner that avoids alayer of air, or some other material of a relatively low refractiveindex, between the multilayer reflector 204 and output coupling element208. Such close optical coupling between the multilayer reflector 204and the output coupling element 208 reduces the possibility of totalinternal reflection of light at the multilayer reflector 204.

The maximum angle of the light within the controlled transmissionmirror, θ_(max), is determined by the relative refractive indices of theinput coupling element 206, n_(i), and the refractive index of theparticular layer of the multilayer reflector 204, n₁, n₂, where thesubscripts 1, 2 refer to the alternating layers in the multilayerreflector 204. Where the input coupling element 206 is a surfacecoupling element, the value of n_(i) is equal to the refractive index ofthe material on which the coupling surface is formed. Propagation fromthe input coupling element 206 into the multilayer reflector 204 issubject to Snell's law. The value of θ_(max) in each alternate layer ofthe multilayer reflector 204 is given by the expression:θ_(max)=sin⁻¹(n _(i) /n _(1,2)).  (2)where either n₁ or n₂ is used. Where n_(i)>n₁ and n_(i)>n₂, then θ_(max)can be up to 90°.

The output coupling element 208 is used to extract at least some of thelight out of the illumination light unit 199. For example, some of light212 may be diffused by the output coupling element 208 so as to pass outof the controlled transmission mirror 120 as light 220.

Other portions of the light, for example ray 222, may not be diverted bythe output coupling element 208. If light 222 is incident at the uppersurface of the output coupling element 208 at an angle greater than thecritical angle of the output coupling element, θ_(c)=sin⁻¹ (1/n₀), whereno is the refractive index of the output coupling element 208 and theoutput coupling element 208 is interfaced with air, then the light 222is totally internally reflected within the output coupling element 208as light 224. The reflected light 224 may subsequently be totallyinternally reflected at the lower surface of the input coupling element206. Alternatively, the light 224 may subsequently be diverted by theinput coupling element 206 and pass out of the controlled transmissionmirror 200 towards the reflecting surface 202.

If the light that passes into the multilayer reflector 204 with an angleof at least θ_(min) is incident at the output coupling element 208 withan angle greater than θ_(c), then that light which is not diverted outof the output coupling element 208 is typically totally internallyreflected within the output coupling element 208. If, however, the lightthat passes into the multilayer reflector 204 with an angle of at leastθ_(min) reaches the output coupling element 208 at a propagation angleless than θ_(c) then a fraction of that light may be transmitted outthrough the output coupling element 208, even without being diverted bythe output coupling element 208, subject to Fresnel reflection loss atthe interface between the output coupling element 208 and the air. Thus,there are many possibilities for the light to suffer multiplereflections and for its direction to be diverted within the reflectingcavity 118. The light may also propagate transversely within the spacebetween the controlled transmission mirror 200 and the reflectingsurface 202. These multiple effects combine to increase the likelihoodthat the light is spread laterally and extracted with to produce abacklight illuminance of more uniform brightness.

Except for the possibility that the multilayer reflector 204 has a valueof Brewster's angle, θ_(B), that is lower than θ_(min), there is aforbidden angular region for light originating at the light source 116a. This forbidden angular region has a half-angle of θ_(min), and islocated above the light source 116 a. Light cannot pass through themultilayer reflector 204 within the forbidden angular region. Light 232from neighboring light sources 116, for example light source 116 b,however, may be able to escape from the controlled transmission mirror200 at a point perpendicularly above light source 116 a, at the axis230, and so the illumination light unit 199 is effective at mixing lightfrom different light sources 116 a, 116 b.

In view of the description of the controlled transmission mirror 200provided above, it can be seen that the function of the input couplingelement 206 is to divert at least some light, that would otherwise beincident at the multilayer reflector 204 at an angle less than θ_(min),so as to be incident at the multilayer reflector 204 at an angle of atleast θ_(min). Also, the function of the output coupling element 208 isto divert at least some light, that would otherwise be totallyinternally reflected within the controlled transmission mirror 200, soas to pass out of the controlled transmission mirror 200.

Another exemplary embodiment of a controlled transmission mirror 200 isschematically illustrated in FIG. 2B, in which a transparent layer 250is disposed between the multilayer reflector 204 and the output couplingelement 208. The transparent layer 250 may be formed of any suitabletransparent material, organic or inorganic, for example polymer orglass. Suitable polymer materials may be amorphous or semi-crystalline,and may include homopolymer, copolymer or blends thereof. Examplepolymer materials include, but are not limited to, amorphous polymerssuch as poly(carbonate) (PC); poly(styrene) (PS); acrylates, for exampleacrylic sheets as supplied under the ACRYLITE® brand by Cyro Industries,Rockaway, N.J.; acrylic copolymers such as isooctyl acrylate/acrylicacid; poly(methylmethacrylate) (PMMA); PMMA copolymers; cycloolefins;cylcoolefin copolymers; acrylonitrile butadiene styrene (ABS); styreneacrylonitrile copolymers (SAN); epoxies; poly(vinylcyclohexane);PMMA/poly(vinylfluoride) blends; atactic poly(propylene); poly(phenyleneoxide) alloys; styrenic block copolymers; polyimide; polysulfone;poly(vinyl chloride); poly(dimethyl siloxane) (PDMS); polyurethanes; andsemicrystalline polymers such as poly(ethylene); poly(propylene);poly(ethylene terephthalate) (PET); poly(carbonate)/aliphatic PETblends; poly(ethylene naphthalate)(PEN); polyamides; ionomers; vinylacetate/polyethylene copolymers; cellulose acetate; cellulose acetatebutyrate; fluoropolymers; poly(styrene)-poly(ethylene) copolymers; PETand PEN copolymers, and clear fiberglass panels. Some of thesematerials, for example PET, PEN and copolymers thereof, may be orientedso as to change the material refractive index from that of the isotropicmaterial. The transparent layer 250 may be used to allow more lateralspreading of the light from the light sources 116 a, 116 b beforeextracting the light from the controlled transmission mirror 200 usingthe output coupling element 208.

One or more of the edges of the transparent layer 250 may be covered bya reflector 252. Thus, light 254 that might otherwise escape from thetransparent layer 250 is reflected back into the transparent layer 250and may be extracted from the illumination light unit 199 as usefulillumination light. The reflector 256 may be any suitable type ofreflector, including a multilayer dielectric reflector, a metal coatingon the edge of the transparent layer 250, a multilayer polymerreflector, a diffuse polymer reflector, or the like. In the illustratedembodiment, the reflector 252 at the side of the transparent layer 250may be also used as a side reflector for the reflecting cavity 114,although this is not intended to be a limitation of the invention.

In another exemplary embodiment, schematically illustrated in FIG. 2C,the transparent layer 250 is disposed between the input coupling element206 and the multilayer reflector 204.

In some other embodiments, the controlled transmission mirror 200 may beprovided with two multilayer reflectors 204, 205 positioned on eitherside of the transparent layer 250, as is schematically illustrated inFIG. 2D. The multilayer reflectors 204, 205 may have the same value ofθ_(min), although this is not required.

The use of a transparent layer is described further in U.S. patentapplication Ser. No. 11/166,723, titled “OPTICAL ELEMENT FOR LATERALLIGHT SPREADING IN BACK-LIT DISPLAYS AND SYSTEM USING SAME” filed oneven date herewith, incorporated herein by reference.

Exemplary embodiments of different types of input coupling elements arenow discussed with reference to FIGS. 3A-3D. In other exemplaryembodiments, not illustrated, a transparent layer may be providedbetween the multilayer reflector and either of the input and outputcoupling elements.

In FIG. 3A, an exemplary embodiment of a controlled transmission mirror320 comprises an input coupling element 326, a multilayer reflector 304and an output coupling element 308. In this particular embodiment, theinput coupling element 326 is a bulk diffusing layer, comprisingdiffusing particles 326 a dispersed within a transparent matrix 326 b.At least some of the light entering the input coupling element 326 at anangle less than θ_(min), for example light rays 328, is scattered withinthe input coupling element 326 at an angle greater than θ_(min), and isconsequently transmitted through the multilayer reflector 304. Somelight, for example ray 330, may not be scattered within the inputcoupling element 326 through a sufficient angle to pass through themultilayer reflector 304, and is reflected by the multilayer reflector304. Suitable materials for the transparent matrix 326 b include, butare not limited to, polymers such as those listed as being suitable foruse in a transparent layer above.

The diffusing particles 326 a may be any type of particle useful fordiffusing light, for example transparent particles whose refractiveindex is different from the surrounding polymer matrix, diffuselyreflective particles, or voids or bubbles in the matrix 326 b. Examplesof suitable transparent particles include solid or hollow inorganicparticles, for example glass beads or glass shells, solid or hollowpolymeric particles, for example solid polymeric spheres or polymerichollow shells. Examples of suitable diffusely reflecting particlesinclude particles of titanium dioxide (TiO₂), calcium carbonate (CaCO₃),barium sulphate (BaSO₄), magnesium sulphate (MgSO₄) and the like. Inaddition, voids in the matrix 426 b may be used for diffusing the light.Such voids may be filled with a gas, for example air or carbon dioxide.

Another exemplary embodiment of a controlled transmission mirror 340 isschematically illustrated in FIG. 3B, in which the input couplingelement 346 comprises a surface diffuser 346 a. The surface diffuser 346a may be provided on the bottom layer of the multilayer reflector 304 oron a separate layer attached to the multilayer reflector 304. Thesurface diffuser 346 a may be molded, impressed, cast or otherwiseprepared.

At least some of the light incident at the input coupling element 346,for example light rays 348, is scattered by the surface diffuser 346 ato propagate an angle greater than θ_(min), and is consequentlytransmitted through the multilayer reflector 304. Some light, forexample ray 350, may not be scattered by the surface diffuser 346 athrough a sufficient angle to pass through the multilayer reflector 304,and is reflected.

Another exemplary embodiment of a controlled transmission mirror unit360 is schematically illustrated in FIG. 3C, in which the input couplingelement 366 comprises a microreplicated structure 367 having facets 367a and 367 b. The structure 367 may be provided on the bottom layer ofthe multilayer reflector 304 or on a separate layer attached to themultilayer reflector 304. The structure 367 is different from thesurface diffuser 346 a of FIG. 3B in that the surface diffuser 346 aincludes a mostly random surface structure, whereas the structure 367includes more regular structures with defined facets 367 a, 367 b.

At least some of the light incident at the input coupling element 366,for example rays 368 incident on facets 367 a, would not reach themultilayer reflector 304 at an angle of θ_(min) but for refraction atthe facet 367 a. Accordingly, light rays 368 are transmitted through themultilayer reflector 304. Some light, for example ray 370, may berefracted by facet 367 b to an angle less than θ_(min), and is,therefore, reflected by the multilayer reflector 304.

Another exemplary embodiment of a controlled transmission mirror 380 isschematically illustrated in FIG. 3D, in which the input couplingelement 386 has surface portions 382 in optical contact with themultilayer reflector 304 and other surface portions 384 that do not makeoptical contact with the multilayer reflector 304, with a gap 388 beingformed between the element 386 and the multilayer reflector 304. Thepresence of the gap 388 provides for total internal reflection (TIR) ofsome of the incident light. This type of coupling element may bereferred to as a TIR input coupling element.

At least some of the light incident at the input coupling element 386,for example rays 390 incident on the non-contacting surface portions 384would not reach the multilayer reflector 304 at an angle of θ_(min) butfor internal reflection at the surface 384. Accordingly, light rays 390may be transmitted through the multilayer reflector 304. Some light, forexample ray 392, may be transmitted through the contacting surfaceportion 382 to the multilayer reflector 304 and is incident at themultilayer reflector 302 at an angle less than θ_(min). The light 392 isreflected by the multilayer reflector 304.

Other types of TIR coupling elements are described in greater detail inU.S. Pat. No. 5,995,690, incorporated herein by reference.

Other types of input coupling elements may be used in addition to thosedescribed in detail here, for example input coupling elements thatinclude a surface or a volume hologram. Also, an input coupling elementmay combine different approaches for diverting light. For example, aninput coupling element may combine a surface treatment, such as asurface structure or surface scattering pattern or surface hologram,with bulk diffusing particles.

It may be desired in some embodiments for the refractive index of theinput coupling element and output coupling element to have a relativelyhigh refractive index, for example comparable to or higher than theeffective refractive index (the average of the refractive indices of thehigh index and low index layers) of the multilayer reflector 304. Ahigher refractive index for the input and output coupling elements helpsto increase the angle at which light may propagate through themultilayer reflector 304, which leads to a greater bandedge shift. This,in turn, may increase the amount of short wavelength light that passesthrough the controlled transmission mirror, thus making the color of thebacklight illumination more uniform. Examples of suitable highrefractive index polymer materials that may be used for input and outputcoupling elements include biaxially stretched PEN and PET which,depending on the amount of stretch, can have in-plane refractive indexvalues of 1.75 and 1.65 respectively for a wavelength of 633 nm.

Commensurate with the choice of materials for the input and outputcoupling elements, the substrate should be chosen to have an index thatdoes not cause TIR that would block prohibitive amounts of lightentering or exiting at large angles. Conversely, a low index for thesubstrate would result in high angles of propagation in the substrateafter injection from the input coupler having a higher index than thesubstrate. These two effects can be chosen to optimize the performanceof the system with respect to color balance and lateral spreading of thelight.

Similar approaches may be used for the output coupling element. Forexample, a controlled transmission mirror unit 420 is schematicallyillustrated in FIG. 4A to have an input coupling element 406, amultilayer reflector 404 and an output coupling element 428. In thisparticular embodiment, the output coupling element 428 is a bulkdiffusing layer, comprising diffusing particles 428 a dispersed within atransparent matrix 428 b. Suitable materials for use as the diffusingparticles 428 a and the matrix 428 b are discussed above with respect tothe input coupling element 326 of FIG. 3A.

At least some of the light entering the output coupling element 428 fromthe multilayer reflector 404, for example light ray 430, may bescattered by the diffusing particles 428 a in the output couplingelement 408 and is consequently transmitted out of the light outputcoupling element 428. Some light, for example ray 432, may not bescattered within the output coupling element 428 and is incident at thetop surface 429 of the output coupling element 428 at an incident angleof θ. If the value of θ is equal to or greater than the critical angle,θ_(c), for the material of the matrix 428 b, then the light 432 istotally internally reflected at the surface 429.

Another exemplary embodiment of controlled transmission reflector 440 isschematically illustrated in FIG. 4B, in which the output couplingelement 448 comprises a surface diffuser 448 a. The surface diffuser 448a may be provided on the upper surface of the multilayer reflector 404or on a separate layer attached to the multilayer reflector 404.

Some light propagating within the multilayer reflector 404, for examplelight 450, is incident at the surface diffuser 448 a and is scatteredout of the light mixing layer 440. Some other light, for example light452, may not be scattered by the surface diffuser 448 a. Depending onthe angle of incidence at the surface diffuser 448 a, the light 452 maybe totally internally reflected, as illustrated, or some light may betransmitted out of the controlled transmission mirror 440 while some isreflected back within the multilayer reflector 404.

Another exemplary embodiment of controlled transmission mirror 460 isschematically illustrated in FIG. 4C, in which the output couplingelement 466 comprises a microreplicated structure 467 having facets 467a and 467 b. The structure 467 may be provided on a separate layer 468attached to the multilayer reflector 404, as illustrated, or be integralwith the top surface of the multilayer reflector 404 itself. Thestructure 467 is different from the surface diffuser 448 a in that thesurface diffuser includes a mostly random surface structure, whereas thestructure 467 includes more regular structures with defined facets 467a, 467 b.

Some light propagating within the multilayer reflector 404, for examplelight 470, is incident at the surface diffuser structure 467 and isrefracted out of the controlled transmission mirror 460. Some otherlight, for example light 472, may not be refracted out of the controlledtransmission mirror 460 by the structure 467, but may be returned to themultilayer reflector 404. The particular range of propagation angles forlight to escape from the controlled transmission mirror 460 is dependenton a number of factors, including at least the refractive indices of thedifferent layers that make up the controlled transmission mirror 460 andthe layer 468, as well as the shape of the structure 467.

Another exemplary embodiment of a controlled transmission mirror 480 isschematically illustrated in FIG. 4D, in which the output couplingelement 486 comprises a light coupling tape that has surface portions482 in optical contact with the multilayer reflector 404 and othersurface portions 484 that do not make optical contact with themultilayer reflector 404, forming a gap 488 between the element 486 andthe multilayer reflector 404.

At least some of the light incident at the output coupling element 486,for example light ray 490, is incident at a portion of the multilayerreflector's surface that is not contacted to the output coupling element486, but is adjacent to a gap 488, and so the light 490 is totallyinternally reflected. Some light, for example ray 492, may betransmitted through the contacting surface portion 482, and be totallyinternally reflected at the non-contacting surface portion 484, and sois coupled out of the controlled transmission mirror 480.

Other types of output coupling elements may be used in addition to thosedescribed in detail here. Also, an output coupling element may combinedifferent approaches for diverting light out of the controlledtransmission mirror. For example, an output coupling element may combinea surface treatment, such as a surface structure or surface scatteringpattern, with bulk diffusing particles.

In some embodiments, the output coupling element may be constructed sothat the degree to which light is extracted is uniform across the outputcoupling element. In other embodiments, the output coupling element maybe constructed so that the degree to which light is extracted out of thecontrolled transmission mirror is not uniform across the output couplingelement. For example, in the embodiment of output coupling element 428illustrated in FIG. 4A, the density of diffusing particles 428 a may bevaried across the output coupling element 428 so that a higher fractionof light can be extracted from some portions of the output couplingelement 428 than others. In the illustrated embodiment, the density ofdiffusing particles 428 a is higher at the left side of the outputcoupling element 428. Likewise, for the embodiments of controlledtransmission mirror 440, 460, 480 illustrated in FIGS. 4B-4D, the outputcoupling elements 448, 468, 488 may be shaped or designed so that ahigher fraction of light can be extracted from some portions of theoutput coupling elements 448, 468, 488 than from other portions. Theprovision of non-uniformity in the extraction of the light from thecontrolled transmission mirror, for example extracting a smallerfraction of light from portions of the controlled transmission mirrorthat contain more light and extracting a higher fraction of light fromportions of the controlled transmission mirror that contain less light,may result in a more uniform brightness profile in the illuminationlight propagating towards the display panel.

The number of bounces made by light within the controlled transmissionmirror, and therefore, the uniformity of the extracted light, may beaffected by the reflectivity of both the input coupling element and theoutput coupling element. The trade-off for uniformity is brightness losscaused by absorption in the input coupling element, the multilayerreflector and the output coupling element. This absorption loss may bereduced by proper choice of materials and material processingconditions.

In some exemplary embodiments, the controlled transmission mirror may bepolarization sensitive, so that light in one polarization state ispreferentially extracted. A cross-section through one exemplaryembodiment of a polarization sensitive controlled transmission mirror520 is schematically illustrated in FIG. 5A. The controlled transmissionmirror 520 comprises an optional transparent layer 502, a multilayerreflector 504, an input coupling element 506 and a polarizationsensitive output coupling element 528. A three-dimensional coordinatesystem is used here to clarify the following description. The axes ofthe coordinate system have been arbitrarily assigned so that the planeof the controlled transmission mirror 520 lies parallel to the x-yplane, with the z-axis having a direction through the thickness of thecontrolled transmission mirror 520. The lateral dimension shown in FIG.5A is parallel to the x-axis, and the y-direction extends in a directionperpendicular to the drawing.

In some embodiments, the extraction of only one polarization of thelight propagating within the controlled transmission mirror 520 iseffected by the output coupling element 528 containing two materials,for example different polymer phases, at least one of which isbirefringent. In the illustrated exemplary embodiment, the outputcoupling element 528 has scattering elements 528 a, formed of a firstmaterial, embedded within a continuous matrix 528 b formed of a secondmaterial. The refractive indices for the two materials are substantiallymatched for light in one polarization state and remain unmatched forlight in an orthogonal polarization state. Either or both of thescattering elements 528 a and the matrix 528 b may be birefringent.

If, for example, the refractive indices are substantially matched forlight polarized in the x-z plane, and the refractive indices of thefirst and second materials are n₁ and n₂ respectively, the conditionholds that n_(1x)≈n_(1z)≈n_(2x)≈n_(2z), where the subscripts x and zdenote the refractive indices for light polarized parallel to the x andz axes respectively. If n_(1y)≠n_(2y), then light polarized parallel tothe y-axis, for example light 530, may be scattered within the outputcoupling element 528 and pass out of the controlled transmission mirror520. The orthogonally polarized light, for example light ray 532,polarized in the x-z plane, remains substantially unscattered on passingwithin the output coupling element 528 because the refractive indicesfor this polarization state are matched. Consequently, if the light 532is incident on the top surface 529 of the output coupling element 528 atan angle equal to, or greater than, the critical angle, θ_(c), of thecontinuous phase 528 b, the light 532 is totally internally reflected atthe surface 529, as illustrated.

To ensure that the light extracted from the output coupling element 528is well polarized, the matched refractive indices may be preferablymatched to within at least ±0.05, and more preferably matched to within±0.01. This reduces the amount of scatter for one polarization state.The amount by which the light in the y-polarization is scattered isdependent on a number of factors, including the magnitude of the indexmismatch, the ratio of one material phase to the other and the domainsize of the disperse phase. Preferred ranges for increasing the amountby which the y-polarized light is forward scattered within the outputcoupling element 528 include a refractive index difference of at leastabout 0.05, a particle size in the range of about 0.5 μm to about 20 μmand a particle loading of up to about 10% or more.

Different arrangements of a polarization-sensitive output couplingelement are available. For example, in the embodiment of output couplingelement 548, schematically illustrated in FIG. 5B, the scatteringelements 548 a constitute a disperse phase of polymeric particles withina continuous matrix 548 b. Note that this figure shows a cross-sectionalview of the output coupling element 548 in the x-y plane. Thebirefringent polymer material of the scattering elements 548 a and/orthe matrix 548 b may be oriented, for example, by stretching in one ormore directions. Disperse phase/continuous phase polarizing elements aredescribed in greater detail in co-owned U.S. Pat. Nos. 5,825,543 and6,590,705, both of which are incorporated by reference.

Another embodiment of polarization-sensitive output coupling element 558is schematically illustrated in cross-section in FIG. 5C. In thisembodiment, the scattering elements 558 a are provided in the form offibers, for example, polymer fibers or glass fibers, in a matrix 558 b.The fibers 558 a may be isotropic while the matrix 558 b isbirefringent, or the fibers 558 a may be birefringent while the matrix558 b is isotropic, or the fibers 558 a and the matrix 558 b may both bebirefringent. The scattering of light in the fiber-based, polarizationsensitive output coupling element 558 is dependent, at least in part onthe size and shape of the fibers 558 a, the volume fraction of thefibers 558 a, the thickness of the output coupling element 558, and thedegree of orientation, which affects the amount of birefringence.Different types of fibers may be provided as the scattering elements 558a. One suitable type of fiber 558 a is a simple polymer fiber formed ofone type of polymer material that may be isotropic or birefringent.Examples of this type of fiber 558 a disposed in a matrix 558 b aredescribed in greater detail in U.S. patent application Ser. No.11/068,159, incorporated herein by reference. Another example of polymerfiber that may be suitable for use in the output coupling element 558 isa composite polymer fiber, in which a number of scattering fibers formedof one polymer material are disposed in a filler of another polymermaterial, forming a so-called “islands-in-the-sea” structure. Either orboth of the scattering fibers and the filler may be birefringent. Thescattering fibers may be formed of a single polymer material or formedwith two or more polymer materials, for example a disperse phase in acontinuous phase. Composite fibers are described in greater detail inU.S. patent application Ser. Nos. 11/068,157 and 11/068,158, both ofwhich are incorporated by reference.

It will be appreciated that the input coupling element may also bepolarization sensitive. For example, where unpolarized light is incidenton the controlled transmission mirror, a polarization-sensitivescattering input coupling element may be used to scatter light of onepolarization state into the controlled transmission mirror, allowing thelight in the orthogonal polarization state to be reflected by themultilayer reflector back to the base reflector. The polarization of thereflected light may then be mixed before returning to the controlledtransmission mirror. Thus, the input coupling element may permit lightin substantially only one polarization state to enter the controlledtransmission mirror. If the different layers of the controlledtransmission mirror maintain the polarization of the light, thensubstantially only one polarization of light may be extracted from thecontrolled transmission mirror, even if a non-polarization-sensitiveoutput coupling element is used. Both the input and output couplingelements may be polarization sensitive. Any of the polarizationsensitive layers used as an output coupling element may also be used asan input coupling element.

In other embodiments of illumination light unit, particularly suitablefor quasi-point sources such as LEDs, the light sources may be locatedwithin the controlled transmission mirror itself. One exemplaryembodiment of such an approach is schematically illustrated incross-section in FIG. 6A. The controlled transmission mirror 620 has atransparent layer 622, a multilayer reflector 624, and an outputcoupling element 628. The lower surface of the transparent layer 622 isprovided with a diverting layer 626. Side reflectors 632 may be providedaround the edge of the controlled transmission mirror 620. The sidereflectors may be used to reflect any light that propagates out of theperipheral edge of the transparent layer 622.

The diverting layer 626 may comprise a transmissive redirecting layer626 a that redirects light, for example any of the layers discussedabove for use as an input coupling element, including bulk or surfacediffusers or a structured surface. The transmissive redirecting layer626 a may be used with a base reflector 618 that reflects the light thathas been transmitted through the transmissive redirecting layer 626 a.The base reflector 618 may be any suitable type of reflector. The basereflector 618 may be a specular or a diffuse reflector and may be formede.g., from a metalized reflector or a MOF reflector. The base reflector618 may be attached to the transmissive redirecting layer 626 a, asillustrated, or may be separate from the transmissive redirecting layer626 a. The diverting layer 626 is not referred to as an input couplingelement in this embodiment, however, because it is not used for couplingthe light into the controlled transmission mirror 620. Differentconfigurations of the diverting layer 626 are possible. In someexemplary embodiments, for example as is schematically illustrated inFIG. 6B, the diverting layer 626 may simply comprise a diffusereflector.

Light sources 616, for example LEDs, although other types of lightsources may also be used, are arranged so that a light emitting surface616 a at least directly faces the transparent layer 622, or may even berecessed within the transparent layer 622. Thus, the light emittingsurface 616 a is disposed between the diverting layer 626 and themultilayer reflector 624. In this embodiment, light 634 from the lightsources 616 enters the transparent layer 622 without being transmittedthrough the diverting layer 626 located at the lower surface of thetransparent layer 622. A refractive index-matching material, for examplea gel, may be provided between the light emitting surface 616 a and thetransparent layer 622 to reduce reflective losses and increase theamount of light coupled into the transparent layer 622 from the lightsource 616.

The light sources 616 may be arranged on a carrier 617. The carrier 617may optionally provide electrical connections to the light sources 616and may also optionally provide a thermal pathway for cooling the lightsources 616.

Even when the light sources 616 directly inject light into thetransparent layer 622 without passing through an input coupling element,the multilayer reflector 624 still controls the minimum angle, θ_(min),at which light propagating within the transparent layer 622 may exit outof the controlled transmission mirror 620. Some light, exemplified bylight rays 636 and 638, is emitted into the transparent layer 622 fromthe light source 616 at an angle less than θ_(min), and is, therefore,reflected by the multilayer reflector 624. Some of the reflected light,for example ray 636, may be diverted by the diverting layer 626 beforeor after incidence at the base reflector 618 and reflected back into thetransparent layer 622 at an angle greater than θ_(min) as ray 636 a.Consequently, some of the light, e.g., ray 636 a, is diverted into anangular range that permits subsequent transmission through themultilayer reflector 624 after only one reflection from the multilayerreflector 624. Another portion of the reflected light, for example lightray 638, may not be diverted at the diverting layer 626 and is,therefore, reflected from the base reflector 618 at an angle that willresult in another reflection at the multilayer reflector 624.

Some of the light emitted from the light sources 616, exemplified bylight rays 640 and 642, is emitted into the transparent layer 622 fromthe light source 616 a at an angle equal to or greater than θ_(min), andis, therefore, transmitted through the multilayer reflector 624. Some ofthe transmitted light, for example ray 640, may be diverted by theoutput coupling element 628 and transmitted out of the controlledtransmission mirror 620 as light 640 a. Another portion of thetransmitted light, for example ray 642, may pass through the outputcoupling element 628 without being diverted and, if it is incident atthe upper surface 628 a of the output coupling element 628 at an anglegreater than the critical angle, θ_(c), is totally internally reflectedback towards the transparent layer 622.

Some of the light 644 propagating within the transparent layer 622 maybe reflected at the edge reflector 632. The edge reflector 632 may beused to reduce the amount of light escaping from the edge of thetransparent layer 622, and thus reduces losses.

Another embodiment of a controlled transmission mirror 650 isschematically illustrated in FIG. 6C, in which the transparent layer 652also operates as a diverting layer. In this embodiment, the transparentlayer 652 contains some diffusing particles, so that some of the lightpassing therethrough is diverted. In one example, light beam 654, whichpropagates from the light source 616 at an angle less than θ_(min) maybe diverted within the transparent layer 652 so as to be incident on themultilayer reflector 624 at an angle greater than θ_(min). In anotherexample, light beam 656, which is reflected by the multilayer reflector624, may be diverted within the transparent layer 652 so as to bereflected by the base reflector 618 at an angle greater than θ_(min).

In a direct-lit display, the illumination light unit may be configuredas a single panel that is positioned behind the display panel. Inanother exemplary embodiment, schematically illustrated in FIG. 7, thebacklight 700 may include a number of illumination light units 702. Inthe illustrated embodiment the light units 702 are configured as barsand each include a number of light sources 716 a, 716 b, 716 c which maybe located at staggered positions. The illumination light units 702 mayhave different shapes. In addition, the light sources 716 a, 716 b, 716c may produce light of different colors. For example, some light sources716 a may produce red light, while other light sources 716 b producegreen light and other light sources 716 c produce blue light. Thedifferently colored light sources 716 a, 716 b, 716 c may be arranged soas to increase the degree to which the light of different colors ismixed so as to produce mixed light of a desired color uniformity.

Another embodiment of an illumination light unit 800 is shown in FIG.8A, in which a number of light sources 806 are located at the end 810 ofa reflecting cavity 802. In this exemplary embodiment, there is morethan one light source 806 and the cross-sectional shape of thereflecting cavity 802 is rectangular. The light sources 806 may eachgenerate light of the same color or of a different color. In the casewhere different light sources 806 generate light of different colors,the light from each light source 806 is mixed in the reflecting cavity802 with the light from the other light sources 806 so that the lightemerging from the controlled transmission mirror 804 may be a mixedcolor. For example, if there are three light sources 806 producing red,green and blue light respectively, the light emerging from thecontrolled emission mirror 804 may be a white color. The shade of themixed color output light depends, inter alia, on the relative outputpowers of the different light sources and on the spectral properties ofthe controlled transmission mirror 804.

The extraction of light through the controlled transmission mirror 804may be graded along the length of the controlled transmission mirror 804so that the brightness of the light extracted from the illuminationlight unit 800 is relatively uniform along its length.

An exemplary embodiment of a backlight 820 that uses the illuminationunit 800 is schematically illustrated in FIG. 8B. The illumination unit800 is at least partially surrounded by a reflector 822 and ispositioned so that the light 824 emitted from the controlledtransmission mirror 804 is directed towards a lightguide 826. Anoptional brightness enhancing layer 828, for example a prismaticbrightness enhancing layer, may be positioned between the illuminationunit 800 and the lightguide 826. The brightness enhancing layer 828reduces the angular spread of the light entering the lightguide 826 andmay promote lateral spreading in the lightguide 826. Some of the light,for example ray 830, may be reflected by the brightness enhancing layer828. The reflected light 830 may be redirected back towards thelightguide 826 by the controlled transmission mirror 804 or some otherreflector in the illumination unit 800, or by the reflector 822 thatsurrounds the illumination light unit 800.

Another embodiment of an illumination light unit 900 is shown in FIG.9A, in which light sources 906 are located on a face 908 of a reflectingcavity 902 opposing a controlled transmission mirror 904. In thisexemplary embodiment, there is more than one light source 906 and thecross-sectional shape of the reflecting cavity 902 is rectangular. Thelight sources 906 may each generate light of the same color or ofdifferent colors. The reflecting cavity 902 may be used to mix the lightfrom the different light sources 906 so that the intensity profile ofthe light output from the controlled transmission mirror 904 isrelatively uniform. Furthermore, in the case where the light sources 906produce light of different colors, the different colored light is mixedso that the light emerging from the controlled transmission mirror 904is a mixed color. For example, if there are three light sources 906producing red, green and blue light respectively, the light emergingfrom the controlled emission mirror 904 may be a white color. The lightfrom the light sources 906 may be mixed within the reflecting cavity 902so that the brightness of the light extracted from the illuminationlight unit may be relatively uniform.

Another embodiment of an illumination unit 920 is schematicallyillustrated in FIG. 9B, in which the controlled transmission mirror 954is positioned on the top of the reflecting cavity 902. Additional lightsources 906 may be placed around the edge of the reflecting cavity 902.

Another embodiment of an illumination light unit 1000 is schematicallyillustrated in FIGS. 10A and 10B. The unit 1000 has a reflecting cavity1002 that includes a reflector 1008 and a controlled transmission mirror1004. One or more light sources 1006 are provided on a base 1007. Thebase 1007 may be reflective. The base 1007 may also provide electricalconnections for driving the light source 1006 and provide a heatsink forremoving heat from the light source 1006.

Light 1020 from the light sources 1006 is reflected by the reflector1008 towards the controlled transmission mirror 1004. The reflector 1008may have any suitable shape and may be curved (as illustrated) or flat.If the reflector 1008 is curved, the curve may be any suitable type ofcurve, for example, elliptical or parabolic. In the illustratedembodiment, the reflector 1008 is curved in one dimension. The reflector1008 may be any suitable type of reflector, for example a metalizedreflector, or a multilayer dielectric reflector, which includes multiplelayer polymer film (MOF) reflectors. Light that is transmitted throughthe controlled transmission mirror 1004 may be coupled into a lightguide 1012 for back-illuminating a display device. The space 1014 withinthe reflecting cavity 1002 may be filled or may be empty. In embodimentswhere the space 1014 is filled, for example, with a transparent opticalbody, then the reflector 1008 may be attached to the outer surface ofthe body. In other embodiments, there is an empty space between thelight source 1006 and the reflector 1008. Different configurations ofreflective cavities are described further in U.S. patent applicationSer. No. 10/701,201 and, incorporated herein by reference.

The light sources 1006, for example LEDs, may all produce light of thesame color, or different LEDs may produce light of different colors, forexample, red, green and blue. In some exemplary embodiments, an optionalwavelength converter 1022 may be used to change the color of at leastsome of the light 1020. For example, where the light 1020 is blue orultraviolet, the wavelength converter 1022 may be used to convert someof the light to green and/or red light 1024 (dashed lines). A low-passreflector 1026 may be positioned between the controlled transmissionmirror 1004 and the wavelength converter 1022. The low-pass reflector1024 transmits the relatively short wavelength light 1020 from the lightsources 1006 and reflects light 1024 a from the wavelength converter1022 towards the light guide 1012.

In another embodiment, the controlled transmission mirror 1004 may useas an output coupling element a diffuser having a matrix loaded withphosphor particles. In such a configuration, some of the lighttransmitted through the multilayer reflector is converted by thephosphor to light of a different wavelength. Light that is not diffusedor converted by the particles may be totally internally reflected by thematrix layer so as to pass back through the multilayer reflector.

Another embodiment of an illumination unit 1100 that may be used as abacklight for a display device is schematically illustrated in FIG. 11A.In this embodiment, one or more light sources 1106 are disposed betweenfirst and second reflectors 1102, 1104. In some embodiments, the lightsources 1106, which may be LEDs, may emit substantially away from thesecond reflector 1102, in which case an optional curved reflector 1108may be provided to direct the light 1110 along the space between thefirst and second reflectors 1102, 1104. In other embodiments, notillustrated, the light sources 1106 may substantially emit lightsideways, in a direction along the space between the first and secondreflectors 1102, 1104.

The first and second reflectors 1102, 1104 may be specular reflectors,for example ESR film available from 3M Company, St. Paul, Minn. Afolding reflector 1112 is positioned at each end to fold the light 1110into a reflecting cavity formed between the second reflector 1104 and acontrolled transmission mirror 1114. The light 1110 is eventuallydirected out of the unit 1100 through the controlled transmission mirror1114. The first reflector 1102 may be mounted on a base 1116 thatprovides electrical power to the light sources 1106 and may also operateas a thermal sink to remove heat from the light sources 1106.

The light sources 1106 may be arranged in different patterns on thefirst reflector 1102. In the arrangement illustrated in FIG. 11B, whichshows a slice through the unit 1100 between the first and secondreflectors 1102, 1104, the light sources 1106 are arranged in a linearpattern, with the light being directed towards the edges 1120 a, 1120 b.In the arrangement schematically illustrated in FIG. 12C, the lightsources 1106 and reflector 1108 are arranged in a radial pattern, sothat the light is directed radially outwards to the folding reflector1112 situated around the periphery of the first reflector 1102.

An illumination light unit as described herein is not restricted to usefor illuminating a liquid crystal display panel. The illumination lightunit may also be used wherever discrete light sources are used togenerate light and it is desirable to have uniform illumination out of apanel that includes one of more of the discrete light sources. Thus, theillumination light unit may find use in solid state space lightingapplications and in signs, illuminated panels and the like.

The present invention should not be considered limited to the particularexamples described above, but rather should be understood to cover allaspects of the invention as fairly set out in the attached claims.Various modifications, equivalent processes, as well as numerousstructures to which the present invention may be applicable will bereadily apparent to those of skill in the art to which the presentinvention is directed upon review of the present specification. Theclaims are intended to cover such modifications and devices.

1. An optical system, comprising: an image-forming panel having anillumination side; and a backlight unit disposed to the illuminationside of the image-forming panel, the backlight unit comprising: at leastfirst and second light sources, the first light source producing lightat a first wavelength and the second light source producing light at asecond wavelength different from the first wavelength, and a reflectingcavity having at least one reflecting surface and a controlledtransmission mirror, the light from the first and second light sourcesbeing reflected within the reflecting cavity, the controlledtransmission mirror comprising an input coupling element, an outputcoupling element and a first multilayer reflector between the input andoutput coupling elements, the first multilayer reflector beingreflective for normally incident light from the first and second lightsources, the input coupling element redirecting at least some of thelight propagating from the first and second light sources in a directionsubstantially perpendicular to the first multilayer reflector into adirection that is transmitted through the first multilayer reflector. 2.A system as recited in claim 1, wherein the output coupling elementcomprises at least one of a bulk diffuser, a surface diffuser, astructured surface and a totally internally reflecting output couplingelement.
 3. A system as recited in claim 1, wherein the input couplingelement comprises at least one of a bulk diffuser, a surface diffuser, astructured surface and a total internal reflection input couplingelement.
 4. A system as recited in claim 1, wherein the controlledtransmission mirror further comprises a transparent layer disposedbetween the input and output coupling elements.
 5. A system as recitedin claim 4, wherein the transparent layer is disposed between the firstmultilayer reflector and the output coupling element.
 6. A system asrecited in claim 4, wherein the first multilayer reflector is disposedbetween the transparent layer and the output coupling element.
 7. Asystem as recited in claim 4, further comprising a second multilayerreflector disposed between the input and output coupling elements, thetransparent layer being disposed between the first and second multilayerreflectors.
 8. A system as recited in claim 4, further comprising a sidereflector disposed adjacent at least one edge of the transparent layer.9. A system as recited in claim 1, wherein the output coupling elementcouples light out of the backlight unit in substantially only onepolarization state.
 10. A system as recited in claim 9, wherein theoutput coupling element comprises a disperse polymeric phase in acontinuous polymeric matrix, at least one of the disperse polymericphase and the continuous polymeric matrix comprising birefringentpolymeric material.
 11. A system as recited in claim 9, wherein theoutput coupling element comprises fibers disposed within a polymericmatrix, at least one of the fibers and the polymeric matrix comprisingbirefringent polymeric material.
 12. A system as recited in claim 1,wherein the image-forming panel comprises a liquid crystal display (LCD)panel, and further comprising a first polarizer disposed on the viewingside of the LCD panel and a second polarizer disposed on theillumination side of the LCD panel.
 13. A system as recited in claim 1,further comprising a controller coupled to control an image displayed bythe image-forming panel.
 14. A system as recited in claim 1, wherein thefirst and second light sources comprise light emitting diodes (LEDs).15. A system as recited in claim 1, wherein the first light sourcecomprises an LED and the second light source comprises a phosphorilluminated by the LED.
 16. A system as recited in claim 1, wherein thefirst multilayer reflector comprises a polymeric multilayer film.
 17. Asystem as recited in claim 1, further comprising one or more lightmanagement films disposed between the backlight unit and theimage-forming panel.
 18. A system as recited in claim 17, wherein thelight management films comprise at least one of a reflective polarizerand a prismatic brightness enhancing film.
 19. A system as recited inclaim 1, wherein the first light source is capable of generating redlight, the second light source is capable of generating green light andthe backlight unit further comprises a third light source capable ofgenerating blue light.
 20. An illumination light unit, comprising: atleast a first light source capable of generating illumination light at afirst wavelength and a second light source capable of generatingillumination light at a second wavelength different from the firstwavelength; and a reflecting cavity having one or more reflectors and acontrolled transmission mirror disposed at an output of the reflectingcavity, the controlled transmission mirror comprising an input couplingelement, an output coupling element and a first multilayer reflectordisposed between the input and output coupling elements, at least someof the illumination light from the first and second light sources beingreflected within the reflecting cavity by the one or more reflectors andbeing transmitted out of the reflecting cavity through the controlledtransmission mirror.
 21. A unit as recited in claim 20, wherein thecontrolled transmission mirror further comprises a transparent layerdisposed between the input and output coupling elements.
 22. A unit asrecited in claim 20, wherein the first and second light sources compriselight emitting diodes (LEDs).
 23. A unit as recited in claim 20, furthercomprising a third light source capable of generating light at a thirdwavelength different from the first and second wavelengths.
 24. A unitas recited in claim 23, wherein the first, second and third wavelengthsare red, green and blue wavelengths respectively.
 25. A unit as recitedin claim 20, further comprising a light wavelength converter disposed toconvert the wavelength of the illumination light output through thecontrolled transmission mirror.
 26. A unit as recited in claim 20,wherein the reflecting cavity is elongated along a longitudinal axis andhas a first end, and the controlled transmission mirror is on a firstside of the reflecting cavity, substantially parallel to thelongitudinal axis.
 27. A unit as recited in claim 26, wherein the atleast a first light source is disposed at the first end of thereflecting cavity.
 28. A unit as recited in claim 26, wherein the atleast a first light source is disposed on a second side of thereflecting cavity.
 29. A unit as recited in claim 20, wherein thereflecting cavity comprises at least one curved reflector on an opticalpath between the one or more light sources and the controlledtransmission mirror.
 30. A unit as recited in claim 29, wherein the atleast one curved reflector is curved in one dimension only.
 31. A unitas recited in claim 29, wherein the at least one curved reflectorcomprises at least two curved reflectors, each of the curved reflectorsbeing curved in two dimensions.